GCC 14: Werror=dangling-pointer in uu_list.c

[tonyhutter]

System information

Type	Version/Name
Distribution Name	Fedora
Distribution Version	40
Kernel Version	6.8.7-300
Architecture	x86-64
OpenZFS Version	master 1f940de

Describe the problem you're observing

GCC 14 on Fedora 40 gives dangling pointer error.

Describe how to reproduce the problem

make on F40

Include any warning/errors/backtraces from the system logs

  CC       lib/libuutil/libuutil_la-uu_list.lo
In function 'list_walk_init',
    inlined from 'uu_list_walk' at lib/libuutil/uu_list.c:511:3:
lib/libuutil/uu_list.c:419:32: error: storing the address of local variable 'my_walk' in '((uu_list_walk_t *)((char *)lp + 32))[1].ulw_prev' [-Werror=dangling-pointer=]
  419 |                 next->ulw_prev = wp;
      |                 ~~~~~~~~~~~~~~~^~~~
lib/libuutil/uu_list.c: In function 'uu_list_walk':
lib/libuutil/uu_list.c:508:32: note: 'my_walk' declared here
  508 |                 uu_list_walk_t my_walk;
      |                                ^~~~~~~
lib/libuutil/uu_list.c:493:25: note: 'lp' declared here
  493 | uu_list_walk(uu_list_t *lp, uu_walk_fn_t *func, void *private, uint32_t flags)
      |              ~~~~~~~~~~~^~
cc1: all warnings being treated as errors

A naive glance makes me think the my_walk variable being dangled is only used on a debug codepath:

       if (lp->ul_debug || robust) {                                             
              uu_list_walk_t my_walk;                                            
              void *e;                                                           
                                                                                 
              list_walk_init(&my_walk, lp, flags);                               
              while (status == UU_WALK_NEXT &&                                   
                  (e = uu_list_walk_next(&my_walk)) != NULL)                     
                     status = (*func)(e, private);                               
              list_walk_fini(&my_walk);                                          
       } else {

Workaround:

diff --git a/lib/libuutil/uu_list.c b/lib/libuutil/uu_list.c
index 0ca6f0520..659f8db41 100644
--- a/lib/libuutil/uu_list.c
+++ b/lib/libuutil/uu_list.c
@@ -416,8 +416,11 @@ list_walk_init(uu_list_walk_t *wp, uu_list_t *lp, uint32_t flags)
                 */
                wp->ulw_next = next = &lp->ul_null_walk;
                wp->ulw_prev = prev = next->ulw_prev;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdangling-pointer="
                next->ulw_prev = wp;
                prev->ulw_next = wp;
+#pragma GCC diagnostic pop
        }
 }